Discotic liquid crystal has generally a disc-like central mother nucleus and side chains extending radially from the mother nucleus thereof, and recently various studies have been done in the liquid crystal field due to anomalous liquid crystal property coming from its structure. Examples of a compound to be the central mother nucleus of the discotic liquid crystal are exemplified by, for example, benzene derivatives, truxene derivatives, phthalocyanine derivatives, triphenylene derivatives, cyclohexane derivatives, porphyrin derivatives, and the like, and among them, triphenylene derivatives are the compounds which attract attention in recent year because they tend to form a discotic nematic phase, which is effective to form an optical functional device.
Among these triphenylene derivatives, particularly various production processes for 2,3,6,7,10,11-hexahydroxytriphenylene have been reported since before, because suitable side chains can be easily introduced at the positions of six hydroxyl groups and the other reason etc.
Specifically, processes for producing a desired 2,3,6,7,10,11-hexahydroxytriphenylene have been known, by synthesizing firstly chemically stable 2,3,6,7,10,11-hexaalkoxytriphenylene with using 1,2-dialkoxybenzene as a raw material (see, JP-A-7-330650, Synthesis, 477, 1994, etc.), then dealkylating with boron tribromide, hydrogen iodide, and the like (see, JP-A-8-119894, J. Mater. Chem., 1992, 2, 1261, etc.). However, since these processes not only required two steps of trimerization step and dealkylation step, but also had such problems that 1,2-dialkoxybenzene as a raw material was comparatively expensive, and boron tribromide and hydrogen iodide to be used in the dealkylation step were highly corrosive, and the like, these processes were not suitable as an industrial process for producing 2,3,6,7,10,11-hexahydroxytriphenylene.
As a method to solve such problems, a process for producing directly 2,3,6,7,10,11-hexahydroxytriphenylene with using 1,2-dihydroxybenzene as a raw material has been attempted (see, JP-A-9-118642, Synthesis, 477, 1994, etc.). Specifically, in Synthesis, 477, 1994, an iron complex of 2,3,6,7,10,11-hexahydroxytriphenylene has been obtained by reacting catechol in the presence of anhydrous ferric (III) chloride and 9.5-fold moles or more of sulfuric acid. However, it has not been described that 2,3,6,7,10,11-hexahydroxytriphenylene has been isolated from the iron complex. In addition, in JP-A-9-118642, desired 2,3,6,7,10,11-hexahydroxytriphenylene has been obtained by reacting catechol in the presence of ferric (III) chloride hydrate to obtain an iron complex and/or a quinone derivative of 2,3,6,7,10,11-hexahydroxytriphenylene, which been then subjected to reduction treatment. Thus, in these processes, although the problems of productivity and corrosion can be solved because dealkylation step is not required by using catechol as a raw material, the problem of requiring many steps has not been solved because a reduction step is necessary to obtain high-purity 2,3,6,7,10,11-hexahydroxytriphenylene in addition to a trimerization step of catechol. Thus, these processes were not advantageous one as an industrial production process.
Under such circumstance, a development of a production process for synthesizing a high-purity 2,3,6,7,10,11-hexahydroxytriphenylene has been demanded, in which not only inexpensive raw materials can be used but also complicated steps of deprotection such as dealkylation from alkoxy groups in hexaalkoxytriphenylene, and reduction of an iron complex and/or a quinone derivative of hexahydroxytriphenylene are not necessary, and which is thereby more easy and simple.
In addition, recently, as a technology relating to a crystal form of 2,3,6,7,10,11-hexahydroxytriphenylene monohydrate, type A crystal of the monohydrate has been disclosed in WO2005/090275. It has been described that the type A crystal can be obtained by distilling off acetone from a solution of 2,3,6,7,10,11-hexahydroxytriphenylene in mixed solvent of acetone-water under a reduced pressure and a specified temperature condition, and that the crystal form is superior in thermal stability with a thermal decomposition temperature (Td) at about 139° C. In addition, in the WO2005/090275, it has been described that all crystals (type B crystal in WO2005/090275) of 2,3,6,7,10,11-hexahydroxytriphenylene obtained by the well-known production process in the prior art are poor in thermal stability, and that an equipment built-in with the type B crystal is poor in durability and has a disadvantage that it cannot exhibit a desired performance over a long period of time. As obvious from this, the type B crystal obtained by the existing production process did not have satisfactory performance.
Under such circumstance, an improvement from type B crystal of 2,3,6,7,10,11-hexahydroxytriphenylene monohydrate having a poor thermal stability to the one having a thermal stability comparable to at least that of type A crystal of the monohydrate, that is, 2,3,6,7,10,11-hexahydroxytriphenylene monohydrate having a superior thermal stability as well as an establishment of a production process for the compound has been demanded.